1. Field of the Invention
The invention relates generally to the recovery of subterranean deposits and more specifically to methods and systems for controlling the accumulation of liquids in a well.
2. Description of Related Art
Gas wells, especially those in which coal-bed methane is produced, may experience large influxes of water downhole that must be removed by pumping to ensure adequate gas production. The pumping system must be designed to assure the pump can effectively remove the produced water from the well. One design criteria recognizes the issue of gas interference. Gas interference is caused when gas, flowing into the suction of the pump, “interferes” with the volumetric efficiency of the pump. To avoid gas interference problems in vertical wells, pumps are frequently placed in a sump or “rat-hole” below the point where the production fluids enter the well. In this configuration, gravity separation allows the lower density gas phase to rise, while the higher density liquids drop into the rat-hole for removal by the pump.
Most downhole pumping systems are designed to handle only a liquid phase. Referring to FIG. 1, when liquid 112 and gas 114 are co-produced in a well 110, the pumping equipment 118 should be configured such that only liquids enter inlets 122 of the pump 118. When two-phase fluids enter a pump, the gas phase can displace an equivalent volume of liquid, thus causing inefficient volumetric pump efficiency. Further problems can result from the compressible nature of the gas, resulting in “gas lock” of the pumping equipment. In addition, due to the diminished flow of the lubricating and cooling liquid through the pump, increased frictional wear can reduce pump life.
Natural gravity separation of gas and liquids becomes more difficult in horizontal wells. If the pump is located in the horizontal section of the well, gravity separation of the fluid is not feasible. Referring to FIG. 2, occasionally in a well 210 having a substantially horizontal portion 214 and a substantially vertical portion 218, a sump or rat-hole is drilled at some point along a curve 226 between the substantially horizontal portion 214 and the substantially vertical portion 218. Frequently, the rat-hole 222 is drilled near the high angle, or vertical section of the well. A pump 230 is placed within the rat-hole 222 and may be driven by a motor 234 positioned at a surface 238 of the well 210. The motor 234 powers the pump 230 via a drive shaft, or tubing string 242. The pump 230 permits removal of liquids from the rat-hole 222, and the liquids in the rat-hole 222 are generally not entrained with gas due to gravity separation. Although separation of the gas and liquid may be successful at this point, the producing formation is exposed to additional fluid head pressure as the column of fluid must build to the vertical head, H, of the rat-hole junction above that of the producing horizontal bore. In some instances involving pressure sensitive formations, this conflicts with the goal to minimize fluid head against such formations. Alternatively, a rat-hole 246 may be drilled near the low angle, or horizontal section of the well; however, as the inclination at the rat-hole departs from vertical, the liquid-gas phase separation efficiency declines. As such, gas interference may still hinder liquid production from the pump, causing the liquid level to rise and create unwanted head against the producing formation.